As a secondary battery with high energy density, a nonaqueous electrolytic lithium secondary battery using a carbonaceous material as an anode is widely studied. While the demand for a nonaqueous electrolytic lithium secondary battery for use as a power supply of a mobile device is still increasing, a new use of a nonaqueous electrolytic lithium secondary battery as a battery of an electric vehicle such as an electric vehicle (EV) driven by a motor alone and a hybrid electric vehicle (HEV) using a combination of an internal combustion engine and a motor is actively developed.
A popularly-used constituent material of an anode of a lithium secondary battery is a carbon based material, and other examples include materials comprising a metallic element such as Zn, Al and Sn or a metalloid element such as Si, Ge, and Sb. As a carbon based material, non-graphitizable carbon (also called “hard carbon”) having a potential capacity that the discharge capacity per gram of carbon is significantly higher than the theoretical discharge capacity of graphite, 372 mAh/g, is also widely used. Especially as a battery for an electric vehicle, non-graphitizable carbon has drawn great attention from the viewpoint of high input-output characteristics that high electric power is repeatedly supplied and received in a short period of time.
Non-graphitizable carbon using a petroleum-based or coal-based pitch as a raw material is proposed as suitable non-graphitizable carbon for use as a constituent material of an anode of a lithium secondary battery (Patent Documents 1 to 3). Typical, conventional steps of producing a desired anode material for a battery from a pitch material are shown in FIG. 1.
As shown in FIG. 1, in the conventional process, an anode material is produced through the following steps of: melt blending a dicyclic or tricyclic aromatic compound having a boiling point of 200° C. or higher added as an additive to a petroleum-based or coal-based pitch material and molding to obtain a molded pitch (“melt blending and molding step”); extracting the additive from the molded pitch with a solvent having low solubility to the pitch and high solubility to the additive to obtain a porous molded pitch (“extracting and drying step”); oxidizing the porous molded pitch with an oxidizing agent such as air to obtain an infusibilized pitch (“oxidizing step”); heating the infusibilized pitch to 600° C. or 680° C. in an inert gas atmosphere (normal pressure) to remove an organic component (tar component) contained in the infusibilized pitch to obtain a carbon precursor having a low volatile content (“tar removal step”); grinding the carbon precursor to obtain a powdery carbon precursor (“grinding step”); and heat treating the powdery carbon precursor in an inert gas at about 800 to 1500° C. to carbonize the powdery carbon precursor (“heat treatment step”).
It has been proposed that in the tar removal step, the pre-carbonization should be carried out in a nitrogen gas atmosphere at normal pressure at a temperature of 600° C. or 680° C. to thereby obtain a carbon precursor having a volatile content of 2% or lower. However, further studies of this step in more detail are still possible, and it has been desired to obtain a carbon material having further reduced irreversible capacity and improved charge-discharge efficiency.